Coated Paving Slabs ‘Clean’ Air

The German city of Fulda has a new weapon in the perpetual fight against Germany’s notoriously unhealthy air: specially coated paving slabs.

Concentrations of toxic nitrogen oxide in German cities regularly exceed the maximum permitted levels. In 2009, toxic nitrogen oxide levels exceeded the maximum permitted levels at 55% of air monitoring stations in urban areas, the German environment ministry reported.

The paving slabs are coated with titanium dioxide (TiO2), which converts nitrogen oxides and other harmful substances into nitrates. Titanium dioxide, a photocatalyst, uses sunlight to accelerate a naturally occurring chemical reaction.

‘Air Clean’

The “Air Clean” nitrogen oxide-reducing paving slabs were developed by concrete manufacturer F.C. Nüdling Betonelemente, backed by research provided by the Fraunhofer Institute for Molecular Biology and Applied Ecology IME in Schmallenberg.

Fraunhofer is Europe’s largest application-oriented research organization. The work was funded by the German Environment Foundation.

The slabs will be laid the length of Petersberger Straße, where recorded pollution levels topped the annual mean limit of 40 micrograms per cubic meter (µg/m3) last year.

Italian Experiments

“Experiments in Italian cities had already shown that photocatalytic paving slabs can improve the air quality,” said Dr. Monika Herrchen, a scientist at the IME.

“We wanted to see if they would also be effective here in Germany, where we have lower levels of light intensity and fewer hours of sunshine.

“Of course, the more intense the sunshine, the quicker the degradation of harmful substances, so our aim was to identify the formula with the highest photocatalytic efficiency rating.”

Development and Testing

In testing the slabs, F.C. Nüdling produced a range of sample slabs incorporating different surfaces, colors, types of cement and TiO2 contents. Because the degradation rates achieved using standard commercial photocatalytic cement proved unsatisfactory, the company ultimately had to develop its own, more effective formula.

“We were able to verify the effectiveness of the optimized slabs in a variety of tests,” said Herrchen. During an extended time field test, the team recorded nitrogen oxide degradation rates of 20% to 30% in specially created street canyons. Measurements were taken at three meters above the photocatalytic slabs, in variable wind and light conditions.

When the wind was still, the experts recorded degradation rates as high as 70% for both nitrogen monoxide (NO) and nitrogen dioxide (NO2). By contrast, measurements taken at three meters above the Gothaer Platz in Erfurt, which is paved with Air Clean slabs, revealed an average degradation rate of 20% for NO2 and 38% for NO.

Stable Effectiveness

“The slabs also remain stable over the long term,” Herrchen said. “Measurements recorded from 14 to 23 months after they were laid revealed no change from the initial degradation capability.”

“Furthermore, the nitrate that is generated during the conversion process poses absolutely no risk to the environment,” she said. “It runs off into the drainage system, then into a wastewater treatment plant, before finally ending up on a farmer’s field or in the groundwater.”

The maximum possible nitrate concentration traceable to photocatalytic reactions is about 5 milligrams per liter (mg/l), while the maximum permitted concentration of nitrate in groundwater is 50 mg/l.

“All in all,” says Herrchen, “it’s possible to say that Air Clean significantly improves the air quality within a short space of time and thus helps protect the environment.”

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